1. Field of the Invention
The present invention relates to a disc-shaped recording medium eccentricity measuring apparatus and a method thereof, and an apparatus for recording and/or reproducing the disc-shaped recording medium. More particularly, the invention relates to a disc-shaped recording medium eccentricity measuring apparatus for measuring the disc-shaped recording medium on the basis of a tracking error signal and a method thereof, and an apparatus for recording and/or reproducing the disc-shaped recording medium, which is equipped with the eccentricity measuring apparatus.
2. Discussion of the Related Art
Disc recording or reproducing apparatus are equipped with actuators for driving objective lenses for optical heads in accordance with a tracking error signal obtained from track guide information such as rows of pits or grooves for controlling the optical spot tracking. The apparatus also include a sled mechanism for displacing the relative position of the whole of the optical head and the disc surface with respect to the direction of the diameter of the disc.
Methods for the sled mechanism where the whole of the optical head is shifted with respect to the disc, and where a turntable on which the disc is mounted is shifted with respect to a fixed position of the optical head are well known.
There is also a method where a sled error signal is generated by extracting the low frequency component from the tracking error signal by passing it through a low pass filter, amplifying it, and then applying it to a drive motor as a drive signal. The sled error signal is a signal exhibiting the amount of the offset between the whole of the optical head and the objective lens for which the actuator within the optical head drives the tracking.
FIGS. 1a to 1c show the waveforms of those signals. FIG. 1c is the tracking error signal which is supplied to a low pass filter to generate the sled error signal shown in FIG. 1b. The sled drive signal shown in FIG. 1a is then obtained.
The sled error signal in FIG. 1b exhibits the angle of radiation of a light beam applied from the optical head with respect to a disc surface. The sled mechanism should therefore carry out shifting in such a direction that the angle of radiation is vertical and the sled error signal becomes zero.
However, even if the sled drive signal is applied to the sled motor, a point at which the shift of the optical head commences depends on the stationary coefficient of friction of the sled mechanism. As the stationary coefficient of friction disperses depending on the apparatus in accordance with load mass and construction of the sled mechanism etc., it is difficult to control the actual sled operation effectively just using this drive voltage.
For example, in FIG. 1a, if the stationary coefficient of friction is first exceeded so that motion commences at a point in time when the sled drive voltage reaches a voltage S.sub.s, a period between T.sub.1 and T.sub.2 becomes a dead band period where there is no actual sled operation even though a voltage is being applied. Also, design and adjustment is made extremely difficult because this operation starting point disperses.
Further, when the sled mechanism starts the shift of the optical head, as shown by the period from T.sub.2 to T.sub.3 in FIG. 1b, the sled error signal is reduced until it is close to zero, so that when the sled error signal becomes zero, the light beam is applied vertically onto the disc surface. However, if the motion coefficient of friction for the sled mechanism is large, it will stop before the sled error signal becomes zero. This causes that the optical beam to always be applied at an angle which is slightly off from the vertical. As this motion coefficient of friction also disperses, operation stoppage control using the drive signal becomes difficult.
Also, as a voltage is always being applied to the sled motor, the influence of voltage fluctuations going to other circuit parts are ever present and this has a detrimental effect on the equipment as a whole.
As a result of this, this applicant put forward a previous technology in Japanese Patent Application No. 4-288196 where a sled shift pulse is applied to a sled mechanism when the sled error signal exceeds a certain threshold value.
This is as shown in FIGS. 2a to 2d, the sled error signal in FIG. 2b obtained as the low frequency component of the tracking error signal in FIG. 2c is compared with a prescribed threshold value S.sub.TH. When, as in at the times T.sub.7 and T.sub.9, the sled error signal reaches the threshold value S.sub.TH. pulses shown in FIG. 2a are outputted as a drive signal. Here, a pulse voltage Vs is set at a voltage sufficient to overcome the coefficient of friction. The threshold value S.sub.TH is then set at a value which is such that the tracking control for the optical head due to the actuator does not exceed a value of this trailing limit. That is, a drive pulse is applied to the sled mechanism when at a tracking trailing limit or when close to the limit using the actuator, whereby the optical head is shifted.
If a fixed voltage pulse of a voltage which is sufficient to overcome the coefficient of friction is used for the drive signal and the period for which this voltage is applied is set based on the sled error signal, instabilities in the shift operation, which depend on dispersion in the coefficient of friction, can be resolved. As a result, an excellent shift operation can be achieved so that the problems mentioned above may be resolved.
However, in a disc which is scanned by the optical head during a recording or reproducing operation, eccentricity due to the fabrication etc., eccentricity due to errors on the disc chucking mechanism or eccentricity due to chucking shifts caused at the time of loading or generated by disturbances occurs.
As a result of these eccentricities, the sled error signal actually becomes a sine waveform shown in the expanded view of FIG. 2d. The frequency of this waveform is the disc rotation frequency, i.e. one period thereof is the equivalent to one rotation period of the disc.
However, in the case where the shift operation of the optical head is carried out in response to the level of the aforementioned sled error signal, it is difficult to carry out accurate shift operation during the execution of the shift operation determination, that is, the comparison between the level of the sled error signal and the threshold value S.sub.TH, as a result of the effects of level fluctuations due to these eccentricities.
A comparison result is therefore obtained which corresponds to a measurement of the extent of the eccentricity taken with respect to the loaded disc in order to cancel out the effects of this eccentricity.
For example, from these kinds of conditions, the amount of eccentricity for a disc in a disc player etc. can be measured.
A method of measuring this extent of eccentricity is, for example, to half rotate the disc with the tracking servo turned off. At this time, as the position to which the laser spot is applied is fixed with the tracking servo turned off, if there is any eccentricity, the beam spot crosses the track and a traverse signal is therefore detected. The number of tracks which are crossed over, that is, the traverse count number, is then taken as the measurement of the eccentricity value at this time.
However, items such as a disc half rotation detecting means are necessary with this kind of measuring method, which makes the construction complicated. This is not suitable for adoption in public use disc players etc.
Further, this cannot be carried out during operations such as reproduction etc. because the tracking servo has to be turned off. As a result, cases cannot be coped with whereby chucking shifts due to disturbances etc. occur during reproduction etc. or the eccentricity component is generated afresh.